The invention relates to a magnetic bearing control device of a magnetic bearing used for a turbo-molecular pump and the like.
In a turbo-molecular pump or a compressor, a magnetic bearing device having an electromagnet is provided as a bearing for supporting a rotating member. In such a magnetic bearing device, generally, three or more axes including a radial axis and an axial axis are controlled, and the electromagnet is controlled with PID feedback based on a signal of a displacement sensor, so that the rotating member is magnetically levitated without contact.
In a rotating device such as a turbo-molecular pump or compressor, resonance is generated due to a characteristic vibration of the rotating member. Conventionally, in each of the axial axis and radial axis, a notch filter with a specific frequency is generally provided for dealing with the resonance. Further, a characteristic frequency of the rotating member depends on a rotational speed. Accordingly, there is a method to adjust a center frequency of a notch filter according to a change in the characteristic frequency (for example, refer to the Patent Document 1).
Patent Document 1: Japanese Patent Publication (Kokai) No. 05-231428
The turbo-molecular pump has a different characteristic frequency of the axial axis caused by a vibration of a rotor blade in each pump. When the device has a different characteristic frequency, it is necessary to provide the notch filter with a wide range of frequencies according to the characteristic frequency. When the notch filter has a wide range of frequencies, the notch filter tends to have a low reduction gain. As a result, a design margin with respect to the resonance decreases, so that the rotor blade may be easy to oscillate. Moreover, when the rotor has many kinds of blades, the characteristic frequencies tend to have a wide range, thereby making it difficult to use the notch filter in constant number design.
When the center frequency of the notch filter is adjusted according to a rotational speed, it is necessary to adjust the center frequency so that a rotational frequency is in a range of a phase-lead characteristic of the notch filter, i.e., a range slightly higher than the center frequency. Therefore, it is necessary to accurately determine a difference among devices. In an actual case, it is possible that the rotor blades resonate due to the difference in the characteristic frequencies among the devices. When the rotor blades resonate, the rotor blades may contact stator blades, thereby producing dust due to the contact.
In view of the problems described above, an object of the present invention is to provide a magnetic bearing control device for properly controlling a magnetic bearing, thereby preventing resonance.
Further objects and advantages of the invention will be apparent from the following description of the invention.